This invention concerns a method for partially demetallizing a security element, a corresponding security element, a value document having such a security element, and an apparatus for manufacturing such a security element.
Value documents as intended by the invention are, inter alia, bank notes, shares, bonds, deeds, vouchers, checks, air tickets, high-value admission tickets, labels for product authentication, credit cards or cash cards, but also other documents at risk of forgery, such as passports, identification cards or other identity documents.
Value documents, in particular bank notes, are usually produced from paper substrates, polymer substrates or combinations of paper and polymer which have particular security features, such as a security thread at least partly incorporated into the paper, or a watermark. Further security features may be so-called window foils, security threads or security bands that are bonded/laminated to the value document or incorporated therein. Security elements usually comprise a polymer or a polymer composition as a carrier material or base material. Typically, security elements have optically variable security features such as holograms or certain color-shift effects to thereby guarantee better anti-forgery security. The particular advantage of optically variable security elements is that the security features on said security elements cannot be imitated by mere copying using a copying machine, since effects of an optically variable security feature are lost or even only appear black through copying.
In existing value documents with optically variable security elements having for example a plurality of optical effects side by side, it is disadvantageous, however, that with increasing complexity in design their manufacture is very time-consuming and cost-intensive. For example, when manufacturing a security element having, side by side, a hologram and very fine, small-structured transparent or partly demetallized regions (such as negative patterns or negative text) at the same time, the manufacture of the demetallized regions is technically very demanding and, on the other hand, a limitation of the fineness of the demetallized regions is pre-specified or bounded by the employed method for demetallization.
In typical manufacturing methods for a security element having different optical effects, such as a hologram and negative patterns, arranged side by side, the manufacture involves a carrier material first being metallized over the full area, and the regions that are to have the negative patterns being demetallized again in a further method step. Since the structure size of the negative patterns frequently lies in the range of 20 μm to 80 μm, the negative patterns can no longer be incorporated via a mask during the metallization operation, but must be manufactured separately.
To enable such partial demetallizing to be performed especially selectively and accurately, the following procedure is known from the prior art:
A substrate is furnished with a grating having a high aspect ratio, for example 0.4, with the aspect ratio being defined by the ratio of structure depth to structure width. The total substrate and thus also the grating are metal-vapor-coated. The metal layer is distinctly thinner in the region of such gratings compared with a metal layer on smooth regions, on account of the enlarged surface. Hence, the metal can be removed either by laser irradiation or by etching more easily at these places than at the smooth surfaces. Therefore, by means of this procedure the substrate can be partially demetallized in the region of the grating while the other regions—in particular, regions that are flatter than those of the grating—remain metallized. The demetallized grating thus renders a negative pattern which is arranged beside metallized regions. This kind of demetallization therefore necessitates a combination of thinned metal layer and a “light trap,” which are both produced by high aspect ratios. Such a method is known for example from EP 1 846 253 or EP 1 843 901. This method is based on the local light absorption, which can be computed by the effective-medium theory.
The requirement to supply optical effects, patterns with ever greater accuracy and finer structuring and complexity results in the situation that finer structures require the embossing and metallizing of ever finer gratings on the substrate. Even now, the requirements for the positional accuracy of patterns and the stroke width of patterns are less than 20 μm in many applications.
Although the embossing and the metallizing of fine gratings are technically mastered, the effort and the risk of defects in the end product increase the finer and deeper the structures become. When such fine gratings are embossed the quality of the cast structure becomes poorer the higher the aspect ratio of the grating is and the finer, smaller the grating structure becomes. Further, in roll-to-roll embossing machines the embossing speed depends on the aspect ratio of the structures to be embossed. Consequently, the manufacturing speed decreases with increasing aspect ratio and fineness of the gratings.